Selecting a Strain Solution
Things to Consider

Specimen Attributes

There is not one extensometer that will work perfectly for every material. Material properties vary greatly, and a large extensometer used to measure strain on rebar can't be used to measure strain for paper. If you buy an extensometer focusing just on specifications you may find that you own an excellent extensometer that can't be used for your test.

Questions to Consider

Testing Environments

Materials properties are influenced by their environment and full characterisation of material properties requires testing under the appropriate environment conditions. The testing conditions will be matched to the environment that the materials will experience in use; e.g. high temperatures for engine components or body temperature saline fluid for implantable medical devices.

Fluids

Testing in fluids involves testing in a fluid bath. In some cases it is possible to immerse contacting extensometers in the fluid, however it is generally better to employ a non-contacting extensometer as this keeps the measurement system away from the damaging effects of the fluid. Use of a non-contact extensometer requires a window in the fluid bath.

Other

Other environments are used in testing, examples include: humidity, salt solution spray, vacuum and gases. Selecting a strain measurement solution for testing in such specialized environments requires careful consideration of the total system.

Temperature

The most common testing environment is low or high temperature. Test temperatures in the range -150 to 600 °C can be achieved using testing chambers employing liquid nitrogen cooling and electrical heating. Strain gauged clip-on extensometers can be used inside a chamber at up to 200 °C, testing at higher temperatures requires extensometers with capacitive sensors. An alternative approach is to use a non-contacting extensometer “looking” through a window in the chamber.

Testing at temperatures above 600 °C is usually performed using tubular furnaces and strain measurement is performed by using ceramic or high temperature metallic rods to transmit the specimen displacement to sensors located outside the furnace.